Jackson Mapping List of String or simple String

In addition to global configuration already mentioned, it is also possible to support this on individual properties:

public class Container {
  @JsonFormat(with = JsonFormat.Feature.ACCEPT_SINGLE_VALUE_AS_ARRAY)
  // ... could also add Feature.WRITE_SINGLE_ELEM_ARRAYS_UNWRAPPED
  public List<String> tags;
}

I have tried this with just jackson outside Spring and it works as expected with:

ObjectMapper mapper = new ObjectMapper();
mapper.enable(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY);

Mind that RestTemplate registers a MappingJacksonHttpMessageConverter with it's own ObjectMapper. Check this answer for how to configure this ObjectMapper.

Since it is a List of keys it will work. if in case property has single value and not in array like below DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY will ensure deserializing single property as a array

{
    "CategoriesKeys":{
    "categoryKey":{
        "keys":"1"
        }
    }
}



@JsonRootName("CategoriesKeys")
    protected static class CategoriesKeys{

        private CategoryKey categoryKey;
//getters and setters 

}

protected static class CategoryKey{

        private List<String> keys;
//getters and setters 

}

TestClass: 

ObjectMapper mapper=new ObjectMapper();
    mapper.configure(SerializationFeature.WRAP_ROOT_VALUE, true);
    mapper.configure(DeserializationFeature.UNWRAP_ROOT_VALUE, true);
    mapper.configure(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY, true);

Output: 

{"CategoriesKeys":{"categoryKey":{"keys":["1"]}}}